The present invention concerns a brewing device for a coffee machine comprising a vertically disposed brewing cylinder having a cylindrical bore and upper and lower pistons mounted within and closing the cylindrical bore to form a brewing chamber, the brewing cylinder and the lower piston being displaceable relative to one another and the upper piston and a process for producing coffee with the brewing device.
Coffee is prepared in automatic coffee machines according to various processes which can be divided into two basic groups. One can distinguish between a first process, in which the hot water passes through the coffee powder located in a filter solely by the force of gravity, and a second process in which the hot water is forced through the coffee powder under pressure (espresso or pressure brewing). The present invention relates in particular to last-mentioned pressure brewing process. Various devices are known in the art for carrying out this process automatically. Depending upon their area of use, e.g. in the household or in the catering industry, these prior art coffee machines differ in construction, particularly with regard to robustness, flexiblility of process steps, and ease of operation and maintenance, etc.
U.S. Pat. No. 4,457,216 describes an automatic coffee machine having two pistons driven independently of each other to carry out various operations on the coffee powder inserted in the brewing chamber. These independently driven pistons make it possible to vary the order of the process steps and adapt the volume of the brewing chamber to an amount of coffee powder being processed in each instance. Hydraulic energy is used to drive the two pistons as well as other auxiliary parts, such as a device for automatically removing the leachedout coffee grinds after brewing. While the use of hydraulic drive means does allow a relatively free selection of relative motion among the individual parts, and thus a relatively free selection of the order of the process steps, the construction of the drive means is relatively time-consuming and expensive. Production of the coffee machine can be profitable only with a high rate of utilization, which is the reason why, as a rule, machines of this type remain reserved for commercial use only.
U.S. Pat. No. 4,852,472 discloses a coffee preparation device having a brewing cylinder with a cylindrical bore and two pistons, by means of which the cylindrical bore can be closed off to form a brewing chamber. The brewing cylinder and the pistons are displaceable relative to each other along the brewing cylinder axis. The brewing device is intended for horizontal operation. The outer surface of the brewing cylinder has an opening for the supply of coffee powder. The powder first falls on the inner surface of the brewing cylinder. It is only when the latter is displaced, a first piston being dragged along, past a second piston, that the previously inserted coffee powder in the reducing chamber is gradually distributed more or less uniformly over the entire cross-section. In order to obtain a layer of coffee powder with a distribution as uniform as possible between the two pistons, the coffee powder is initially pressed at a certain pressure. To this end, the second piston, acting against a spring, is disposed displaceably with respect to a fixed machine frame. The desired initial pressing pressure is reached when the second piston has compressed the spring a fixed amount. At this point the advance of the brewing cylinder is stopped and the brewing process is started. When the brewing process is finished, the brewing cylinder is driven back toward its original position. The second piston releases one end of the brewing cylinder. Depending on how the brewing cylinder moves relative to the first piston as a result of the effects of friction, the leached-out coffee powder then falls out of the brewing cylinder when the first piston has reached the brewing cylinder end released by the second piston. At the latest, this will happen when the brewing cylinder has reached the completely driven back position, after the first piston has been halted previously by a stop. The stop is designed so that when the brewing cylinder moves forward again into the filling position, the first piston is held immobile until released by the advancing brewing cylinder.
Although the coffee preparation device disclosed here is of simple construction, it does have some disadvantages. Because of the horizontal arrangement, the coffee powder must be brewed with pressing pressure so that a more or less uniform distribution over the cross-section of the brewing chamber is guaranteed and even brewing of the coffee powder is obtained. However, experience has shown that the quality of coffee brewed in loose powder form is better.